Insulated protective coating for mitigation of stress corrosion cracking of metal components in high-temperature water

ABSTRACT

A method for mitigating crack initiation and propagation on the surface of metal components in a water-cooled nuclear reactor. An electrically insulating coating is applied on the surfaces of IGSCC-susceptible reactor components. The preferred electrically insulating material is yttria-stabilized zirconia. The presence of an electrically insulating coating on the surface of the metal components shifts the corrosion potential in the negative direction without the addition of hydrogen and in the absence of noble metal catalyst. Corrosion potentials ≦-0.5 V she  can be achieved even at high oxidant concentrations and in the absence of hydrogen.

FIELD OF THE INVENTION Field of the Invention

This invention relates to reducing the corrosion potential of componentsexposed to high-temperature water. As used herein, the term"high-temperature water" means water having a temperature of about 100°C. or greater, steam, or the condensate thereof. High-temperature watercan be found in a variety of known apparatus, such as water deaerators,nuclear reactors, and steam-driven power plants.

BACKGROUND OF THE INVENTION

Nuclear reactors are used in electric power generation, research andpropulsion. A reactor pressure vessel contains the reactor coolant, i.e.water, which removes heat from the nuclear core. Respective pipingcircuits carry the heated water or steam to the steam generators orturbines and carry circulated water or feedwater back to the vessel.Operating pressures and temperatures for the reactor pressure vessel areabout 7 MPa and 288° C. for a boiling water reactor (BWR), and about 15MPa and 320° C. for a pressurized water reactor (PWR). The materialsused in both BWRs and PWRs must withstand various loading, environmentaland radiation conditions.

Some of the materials exposed to high-temperature water include carbonsteel, alloy steel, stainless steel, and nickel-based, cobalt-based andzirconium-based alloys. Despite careful selection and treatment of thesematerials for use in water reactors, corrosion occurs on the materialsexposed to the high-temperature water. Such corrosion contributes to avariety of problems, e.g., stress corrosion cracking, crevice corrosion,erosion corrosion, sticking of pressure relief valves and buildup of thegamma radiation-emitting Co-60 isotope.

Stress corrosion cracking (SCC) is a known phenomenon occurring inreactor components, such as structural members, piping, fasteners, andwelds, exposed to high-temperature water. As used herein, SCC refers tocracking propagated by static or dynamic tensile stressing incombination with corrosion at the crack tip. The reactor components aresubject to a variety of stresses associated with, e.g., differences inthermal expansion, the operating pressure needed for the containment ofthe reactor cooling water, and other sources such as residual stressfrom welding, cold working and other asymmetric metal treatments. Inaddition, water chemistry, welding, crevice geometry, heat treatment,and radiation can increase the susceptibility of the metal in acomponent to SCC.

It is well known that SCC occurs at higher rates when oxygen is presentin the reactor water in concentrations of about 1 to 5 parts per billion(ppb) or greater. SCC is further increased in a high radiation fluxwhere oxidizing species, such as oxygen, hydrogen peroxide, andshort-lived radicals, are produced from radiolytic decomposition of thereactor water. Such oxidizing species increase the electrochemicalcorrosion potential (ECP) of metals. Electrochemical corrosion is causedby a flow of electrons from anodic to cathodic areas on metallicsurfaces. The ECP is a measure of the thermodynamic tendency forcorrosion phenomena to occur, and is a fundamental parameter indetermining rates of, e.g., SCC, corrosion fatigue, corrosion filmthickening, and general corrosion.

Corrosion potential has been clearly shown to be a primary variable incontrolling the susceptibility to SCC in BWR environments. FIG. 1 showsthe observed and predicted crack growth rate as a function of corrosionpotential for furnace-sensitized Type 304 stainless steel at 27.5 to 30MPa√m in 288° C. water over the range of solution conductivities from0.1 to 0.5 μS/cm. Data points at elevated corrosion potentials andgrowth rates correspond to irradiated water chemistry conditions in testor commercial reactors.

Corrosion (or mixed) potential represents a kinetic balance of variousoxidation and reduction reactions on a metal surface placed in anelectrolyte, and can be decreased by reducing the concentration ofoxidants such as dissolved oxygen. FIG. 2 is a schematic of E(potential) vs. log |i| (absolute value of current density) curvesshowing the interaction of H₂ and O₂ on a catalytically active surfacesuch as platinum or palladium. i₀ is the exchange current density, whichis a measure of the reversibility of the reaction. Above i₀, activationpolarization (Tafel behavior) is shown in the sloped, linear regions.i_(L) represents the limited current densities for oxygen diffusion tothe metal surface, which vary with mass transport rate (e.g., oxygenconcentration, temperature, and convection). The corrosion potential inhigh-temperature water containing oxygen and hydrogen is usuallycontrolled by the intersection of the O₂ reduction curve (O₂ +2H₂ O+4e⁻→4OH⁻) with the H₂ oxidation curve (H₂ →2H⁺ +2e⁻), with the low kineticsof metal dissolution generally having only a small role.

The fundamental importance of corrosion potential versus, e.g., thedissolved oxygen concentration per se, is shown in FIG. 3, where thecrack growth rate of a Pd-coated CT specimen drops dramatically onceexcess hydrogen conditions are achieved, despite the presence of arelatively high oxygen concentration. FIG. 2 is a plot of crack lengthvs. time for a Pd-coated CT specimen of sensitized Type 304 stainlesssteel showing accelerated crack growth at ≈0.1 μM H₂ SO₄ in 288° C.water containing about 400 ppb oxygen. Because the CT specimen wasPd-coated, the change to excess hydrogen caused the corrosion potentialand crack growth rate to drop.

In a BWR, the radiolysis of the primary water coolant in the reactorcore causes the net decomposition of a small fraction of the water tothe chemical products H₂, H₂ O₂, O₂ and oxidizing and reducing radicals.For steady-state operating conditions, approximately equilibriumconcentrations are established for O₂, H₂ O₂, and H₂ in the water whichis recirculated and for O₂ and H₂ in the steam going to the turbine.These concentrations of O₂, H₂ O₂, and H₂ are oxidizing and result inconditions that can promote intergranular stress corrosion cracking(IGSCC) of susceptible materials of construction. One method employed tomitigate IGSCC of susceptible material is the application of hydrogenwater chemistry (HWC), whereby the oxidizing nature of the BWRenvironment is modified to a more reducing condition. This effect isachieved by adding hydrogen gas to the reactor feedwater. When thehydrogen reaches the reactor vessel, it reacts with the radiolyticallyformed oxidizing species homogeneously and on metal surfaces to reformwater, thereby lowering the concentration of dissolved oxidizing speciesin the water in the vicinity of metal surfaces. The rate of theserecombination reactions is dependent on local radiation fields, waterflow rates and other variables.

The injected hydrogen reduces the level of oxidizing species in thewater, such as dissolved oxygen, and as a result lowers the ECP ofmetals in the water. However, factors such as variations in water flowrates and the time or intensity of exposure to neutron or gammaradiation result in the production of oxidizing species at differentlevels in different reactors. Thus, varying amounts of hydrogen havebeen required to reduce the level of oxidizing species sufficiently tomaintain the ECP below a critical potential required for protection fromIGSCC in high-temperature water. As used herein, the term "criticalpotential" means a corrosion potential at or below a range of values ofabout -0.230 to -0.300 V based on the standard hydrogen electrode (SHE)scale. IGSCC proceeds at an accelerated rate in systems in which the ECPis above the critical potential, and at a substantially lower or zerorate in systems in which the ECP is below the critical potential (seeFIG. 1). Water containing oxidizing species such as oxygen increases theECP of metals exposed to the water above the critical potential, whereaswater with little or no oxidizing species present results in an ECPbelow the critical potential.

Initial approaches to reduce the corrosion potential focused onrelatively large additions of dissolved hydrogen, which proved capableof reducing the dissolved oxygen concentration in the water outside ofthe core from ≈200 ppb to <5 ppb, with a resulting change in corrosionpotential from ≈+0.05 V_(she) to ≦-0.25 V_(she). This approach is termedhydrogen water chemistry, and is in commercial use in domestic andoverseas BWRs. Corrosion potentials of stainless steels and otherstructural materials in contact with reactor water containing oxidizingspecies can usually be reduced below the critical potential by injectionof hydrogen into the feedwater. For adequate feedwater hydrogen additionrates, conditions necessary to inhibit IGSCC can be established incertain locations of the reactor. Different locations in the reactorsystem require different levels of hydrogen addition. Much higherhydrogen injection levels are necessary to reduce the ECP within thehigh radiation flux of the reactor core, or when oxidizing cationicimpurities, e.g., cupric ion, are present.

It has been shown that IGSCC of Type 304 stainless steel (containing18-20% Cr, 8-10.5% Ni and 2% Mn) and all other structural materials usedin BWRs can be mitigated by reducing the ECP of the stainless steel tovalues below -0.230 V(SHE). An effective method of achieving thisobjective is to use HWC. However, high hydrogen additions, e.g., ofabout 200 ppb or greater in the reactor water, that may be required toreduce the ECP below the critical potential, can result in a higherradiation level in the steam-driven turbine section from incorporationof the short-lived N¹⁶ species in the steam. For most BWRs, the amountof hydrogen addition required to provide mitigation of IGSCC of pressurevessel internal components results in an increase in the main steam lineradiation monitor by a factor of five to eight. This increase in mainsteam line radiation can cause high, even unacceptable, environmentaldose rates that can require expensive investments in shielding andradiation exposure control. Thus, recent investigations have focused onusing minimum levels of hydrogen to achieve the benefits of HWC withminimum increase in the main steam radiation dose rates.

An effective approach to achieve this goal is to either coat or alloythe stainless steel surface with palladium or other noble metals. Thepresence of palladium on the stainless steel surface reduces thehydrogen demand to reach the required IGSCC critical potential of -0.230V(SHE). The use of alloys or metal coatings containing noble metalspermits lower corrosion potentials (e.g., ≈-0.5 V_(she)) to be achievedat much lower hydrogen addition rates. For example, U.S. Pat. No.5,135,709 to Andresen et al. discloses a method for lowering the ECP oncomponents formed from carbon steel, alloy steel, stainless steel,nickel-based alloys or cobalt-based alloys which are exposed tohigh-temperature water by forming the component to have a catalyticlayer of a noble metal. Such approaches rely on the very efficientrecombination kinetics of dissolved oxygen and hydrogen on catalyticsurfaces (see the high i_(O) for H₂ oxidation in FIG. 2, which causesmost O₂ reduction curves to intersect at -0.5 V_(she)). This wasdemonstrated not only for pure noble metals and coatings, but also forvery dilute alloys or metal coatings containing, e.g., <0.1 wt. % Pt orPd (see FIGS. 3 to 5). FIG. 4 shows corrosion potential measurements onpure platinum, Type 304 stainless steel and Type 304 stainless steelthermally sprayed by the hyper velocity oxy-fuel (HVOF) technique with apowder of Type 308L stainless steel containing 0.1 wt. % palladium. Datawere obtained in 285° C. water containing 200 ppb oxygen and varyingamounts of hydrogen. The potential drops dramatically to itsthermodynamic limit of ≈-0.5 V_(she) once the hydrogen is near or abovethe stoichiometric value associated with recombination with oxygen toform water (2H₂ +O₂ →2H₂ O) . FIG. 5 shows corrosion potentials of Type304 stainless steel doped with 0.35 wt. % palladium at a flow rate of200 cc/min. in 288° C. water containing up to 5000 ppb oxygen andvarious amounts of hydrogen.

If the surface recombination rate is much higher than the rate of supplyof oxidants to the metal surface (through the stagnant, near-surfaceboundary layer of water), then the concentration of oxidants (at thesurface) becomes very low and the corrosion potential drops to itsthermodynamic limit of ≈-0.5 V_(she) in 288° C. water, even though thebulk concentration of dissolved oxygen remains high (FIGS. 3 to 5).Further, the somewhat higher diffusion rate of dissolved hydrogen versusdissolved oxygen through the boundary layer of water permits somewhatsubstoichiometric bulk concentrations of hydrogen to support fullrecombination of the oxidant which arrives at the metal surface. Whilesome hydrogen addition to BWRs will still be necessary with thisapproach, the addition can be vastly lower than (as low as ≦1% of) thatrequired for the initial hydrogen water chemistry concept. Hydrogenadditions remain necessary since, while oxidants (primarily oxygen andhydrogen peroxide) and reductants (primarily hydrogen) are produced byradiolysis in stoichiometric balance, hydrogen preferentially partitionsto the steam phase in a BWR. Also, no hydrogen peroxide goes into thesteam. Thus, in BWR recirculation water there is some excess of oxygenrelative to hydrogen, and then, in addition, a fairly largeconcentration of hydrogen peroxide (e.g., ≈200 ppb). Approaches designedto catalytically decompose the hydrogen peroxide before or during steamseparation (above the core) have also been identified.

While the noble metal approach works very well under many conditions,both laboratory data and in-core measurements on noble metals show thatit is possible for the oxidant supply rate to the metal surface toapproach and/or exceed the recombination rate (see FIGS. 6 and 7). FIG.6 shows the effect of feedwater hydrogen addition on the corrosionpotential of stainless steel and platinum at several locations at theDuane Arnold BWR. At ≈2 SCFM of feedwater hydrogen addition, thecorrosion potentials in the recirculation piping drop below ≈-0.25V_(she) . However, in the high flux (top of core) regions, even for purePt, the corrosion potential remains above ≈-0.25 V_(she) at feedwaterhydrogen levels of ≧15 scfm, where long-term operation is veryunattractive due to the cost of hydrogen and the increase in volatileN¹⁶ (turbine shine). FIG. 7 shows corrosion potential vs. hydrogenaddition for Pd-coated Type 316 stainless steel in 288° C. water in arotating cylinder specimen, which simulates high fluid flow rateconditions. The water contained 1.0 parts per million (ppm) O₂. As thehydrogen level was increased above stoichiometry, the potentialdecreased, but only to about -0.20 V_(she). The oxygen supply rate inthese tests had exceeded the exchange current density (i_(O)) of thehydrogen reaction (see FIG. 2), and activation polarization (Tafelresponse) of the hydrogen reaction began to occur, causing a shift to amixed (or corrosion) potential which is in between the potentialsmeasured in normal and extreme hydrogen water chemistry on noncatalyticsurfaces.

At the point where the oxidant supply rate to the metal surfaceapproaches and/or exceeds the recombination rate, the corrosionpotential will rapidly increase by several hundred millivolts (e.g., to≧-0.2 V_(she)). Indeed, even under (relatively small) excess hydrogenconditions, pure platinum electrodes in the core of BWRs exhibitcorrosion potentials which are quite high, although still somewhat lowerthan (noncatalytic) stainless steel (see FIG. 6). At very high hydrogenlevels (well above those typically used in the original hydrogen waterchemistry concept), the corrosion potential on noble metal surfaces willdrop to <-0.3 V_(she) (see FIG. 6). However, the huge cost of thehydrogen additions combined with large observed increase in volatileradioactive nitrogen in the steam (i.e., N¹⁶, which can raise theradiation levels in the turbine building) make the use of very highhydrogen addition rates unpalatable.

SUMMARY OF THE INVENTION

The present invention is an alternative method for achieving theobjective of low ECPs which result in slow or no crack growth instainless steel and other metals. This is accomplished by coating thesurfaces of IGSCC-susceptible reactor components with an electricallyinsulating material such as zirconia. In accordance with the presentinvention, the metal corrosion potential is shifted in the negativedirection without the addition of hydrogen and in the absence of noblemetal catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the observed and predicted crack growth rate as a functionof corrosion potential for furnace-sensitized Type 304 stainless steelin 288° C. water.

FIG. 2 is a schematic of E (potential) vs. log i (absolute value ofcurrent density) curves showing the interaction of H₂ and O₂ on acatalytically active surface such as platinum or palladium.

FIG. 3 is a plot of crack length vs. time for a Pd-coated CT specimen ofsensitized Type 304 stainless steel in 288° C. water containing about400 ppb oxygen and 0.1 μM H₂ SO₄.

FIG. 4 is a graph showing corrosion potentials of pure platinum (□),Type 304 stainless steel (◯) and Type 304 stainless steel thermallysprayed by the hyper velocity oxy-fuel (HVOF) technique with a powder ofType 308L stainless steel containing 0.1 wt. % palladium ( ).

FIG. 5 is a graph showing corrosion potentials of Type 304 stainlesssteel doped with 0.35 wt. % palladium at a flow rate of 200 cc/min. in288° C. water containing various amounts of hydrogen and the followingamounts of oxygen: (◯) 350 ppb; ( ) 2.5 ppm; and (□) 5.0 ppm.

FIG. 6 is a graph showing the effect of feedwater hydrogen addition onthe corrosion potential of Type 304 stainless steel at the top of thecore ( ), at the bottom of the core ( ), and in the recirculation piping( ); and of platinum at the top (◯) and bottom (□) of the core.

FIG. 7 is a graph showing corrosion potential vs. hydrogen addition forPd-coated Type 316 stainless steel in 288° C. water in a rotatingcylinder specimen, which simulates high fluid flow rate conditions of0.3 ( ), 1.5 (□) and 3.0 ( ) m/sec.

FIG. 8 is a schematic of electrochemical processes which generally leadto elevated corrosion potentials on the outside (mouth) of a crack andlow corrosion potentials in the inside (tip) of the crack.

FIGS. 9A to 9D provide a schematic comparison of the corrosionpotentials φ_(c) which form under high radiation flux on various coatedand uncoated components.

FIG. 10 is a schematic of an insulated protective coating, depicted asthermally sprayed zirconia powder.

FIGS. 11 and 12 are plots showing the corrosion potential of Type 304stainless steel uncoated ( ) and coated (□) with yttria-stabilizedzirconia by air plasma spraying versus oxygen and hydrogen peroxideconcentration respectively.

FIGS. 13 and 14 are plots showing the corrosion potential versus oxygenconcentration for uncoated Type 304 stainless steel ( ); Type 304stainless steel coated with yttria-stabilized zirconia with thicknessesof 3 mils (□), 5 mils ( ) and 10 mils (◯); and pure zirconium ( ) afterbeing immersed in pure water for 2 days and in water containing variouswater chemistry conditions for 3 months, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a technique for solving the problem ofachieving low corrosion potentials in the high-flux, in-core region (orin other regions which may have very high oxidant supply rates from highconcentrations and/or high fluid flow rates/convection). The techniqueentails the formation of an electrically insulating protective coatingon SCC-susceptible surfaces of metal components of a water-coolednuclear reactor. The insulated protective coating is designed to alterthe balance between the rate of supply of oxidants to the surface andthe rate of recombination on the surface by limiting the supply kinetics(by restricting the mass transport of reactants through a porous,insulated layer). The technique of the present invention is based on thefollowing fundamental considerations.

The first consideration is that corrosion potentials are created only atmetal-water interfaces. Thus, while on a metal coating the corrosionpotential is formed at the interface of the metal coating with the bulkwater, on a porous insulated coating, the corrosion potential is formedat the interface of the substrate metal and the water with which it isin contact (i.e., the water in the pores).

The influence of corrosion potential on stress corrosion crackingresults from the difference in corrosion potential at the generally highpotential crack mouth/free surface versus the always low potential(e.g., -0.5 V_(she)) within the crack/crevice tip. This potentialdifference causes electron flow in the metal and ionic flow in thesolution, which induces an increase in the anion concentration in thecrack.

FIG. 8 is a schematic of electrochemical processes which generally leadto elevated corrosion potentials on the outside (mouth) of a crack andlow corrosion potentials in the inside (tip) of the crack. The potentialdifference Δφ_(c) causes anions A⁻ (e.g., Cl⁻) to concentrate in thecrack, but only if there is both an ionic path and an electron path.

FIGS. 9A to 9D provide a schematic comparison of the corrosionpotentials φ_(c) which form under high radiation flux: (A) on anuncoated (e.g., stainless steel) component (high φ_(c)); (B) on acomponent coated with a catalytic metal coating where the rate of supplyof reactants to the surface is not too rapid (low φ_(c)); (C) on acomponent coated with a catalytic metal coating where the rate of supplyof reactants to the surface approaches or exceeds the H₂ --O₂recombination kinetics (moderate φ_(c)); and (D) on a component coatedwith an insulated protective coating (always at a low corrosionpotential).

Thus, to influence stress corrosion cracking, the elevated crack mouthcorrosion potential must form on a surface that is in electrical contactwith the component of interest. If an insulating coating (see FIGS. 9and 10) were applied to a metal component and some porosity or crackingin the coating is assumed to exist, the corrosion potential would beformed only at the metal component-water interface.

Thus, a crevice would be formed by the coating, but since it iselectrically insulating, the crevice cannot represent an"electrochemical" crevice, but only a "restricted mass transport"geometry. The critical ingredient in "electrochemical" crevices is thepresence of a conducting material in simultaneous contact with regionsof high potential (e.g., a crack mouth) and regions of low potential(e.g., a crack tip). Thus, it would not help to have a component coveredby an insulating layer, which layer is in turn covered by a metal layer(or interconnected metal particles) within which exists a crevice orcrack. Under these conditions, the aggressive crevice chemistry couldform in the outer metal layer, which in turn would be in contact withthe component.

The second consideration is that if the insulated coating is impermeableto water, then obviously there can be neither corrosion potential formedon the underlying metal nor concern for stress corrosion cracking. Anypores or fine cracks in an insulating layer provide highly restrictedmass transport and thus are equivalent to a very thick near-surfaceboundary layer of stagnant water. Since oxidants are always beingconsumed at metal surfaces, this very restricted mass transport (reducedrate of oxidant supply) causes the arrival rate of oxidants through theinsulated coating to the substrate to decrease below the rate of theirconsumption. Under these mass transport limiting circumstances, thecorrosion potential rapidly decreases to values ≦-0.5 V_(she), even forhigh oxidant concentrations and in the absence of stoichiometric excesshydrogen (or any hydrogen). Numerous observations consistent with thishave been made, including low potentials on stainless steel surfaces atlow oxygen levels (e.g., 1 to 10 ppb), as well as in (just inside)crevices/cracks, even at very high oxygen levels.

Thus, corrosion potentials -0.5 V_(she) can be achieved even at highoxidant concentrations and, not only in the absence of stoichiometricexcess hydrogen, but also in the absence of any hydrogen. This may proveto be a critical invention for BWR plants which are unable (because ofcost or because of the high N¹⁶ radiation levels from hydrogen addition)to add sufficient hydrogen to guarantee stoichiometric excess hydrogenconditions at all locations in their plant.

While various non-conducting materials could be used, zirconia (ZrO₂) isa good initial choice because it can be thermally sprayed and is verystable in high temperature water, both structurally (e.g., it is notprone to spalling and is not susceptible to environmentally assistedcracking) and chemically (e.g., it does not dissolve or react). Zirconiacan be obtained in various particle sizes, so that there is flexibilityin adjusting the thermal spray parameters. Alumina is also an option.The dissolution rate of alumina in 288° C. water is higher than that forzirconia, but is still very low. Additions of other oxides into thecoating may also be advantageous. For example, ZnO has been shown toyield many benefits in BWRs, including reduced incorporation of Co⁶⁰ infilms (thereby lowering the radiation level, e.g., in piping) andreduced susceptibility to SCC.

FIG. 10 is a schematic of an insulated protective coating, depicted asparticles 4 of zirconia powder which have been thermally sprayed on ametal component surface 2. Due to the insulating nature of zirconia,there is no electrical connection between external (high oxidant) waterand the metal component substrate. Thus, the insulated protectivecoating prevents an electrochemical crevice cell from being formed (seeFIG. 8), while restricting mass transport of oxidants to the underlyingmetal substrate (see FIGS. 2 and 7) to sufficiently low rates such thatthe corrosion potential of the metal component is always low (i.e., -0.5V_(she)).

Preliminary experimental data (shown in FIG. 11) were obtained in 288°C. pure water on a cylindrical stainless steel electrode coated withyttria-stabilized zirconia (YSZ) by air plasma spraying. A Cu/Cu₂ Omembrane reference electrode was used to measure the corrosionpotentials of the stainless steel autoclave, a platinum wire and theYSZ-coated stainless steel specimen. At oxygen concentrations up to ≈1ppm (during BWR operation, the equivalent oxygen concentration (O₂+0.5×H₂ O₂) is about 100 to 600 ppb), the corrosion potential of theYSZ-coated specimen remained at ≦-0.5 V_(she) despite the highpotentials registered on the stainless steel autoclave (+0.20 V_(she))and the platinum electrode (+0.275 V_(she)). This is consistent withnumerous observations of low potentials on stainless steel surfaces atlow oxygen levels (e.g., 1 to 10 ppb) as well as inside crevices/cracks,even at very high oxygen levels.

Similar observations were obtained in hydrogen peroxide, where lowpotentials were observed on the YSZ-coated specimen at concentrationsabove 1 ppm (see FIG. 12). By contrast, uncoated stainless steelexhibited a high corrosion potential of ≈+0.150 V_(she). Low potentialswere also observed on the YSZ-coated specimen in water containing 1 ppmO₂ when the specimen was rotated at 500 rpm, corresponding to 0.7 m/seclinear flow rate. This is not surprising, since the higher flow ratesmerely act to reduce the thickness of the stagnant boundary layer ofliquid, a layer whose thickness is small relative to the zirconiacoating. The success in maintaining low corrosion potentials under theseconditions shows that the electrically insulating zirconia layer greatlyreduces mass transport to the underlying metal surface such that, evenin the absence of catalytic agents such as palladium, the cathodic(oxygen reduction) reaction is mass transport limited just as inuncoated specimens in solutions of very low dissolved oxygen content.

Further corroboration exists in the corrosion potential measurements onZircaloy in 288° C. water, which apparently are always lower than -0.5V_(she), even in aerated solutions. The relatively highly electricallyinsulating nature of the zirconia film causes the corrosion potential tobe formed at the metal surface where the oxidant concentration is verylow due to its restricted transport through the zirconia film.

Additional experimental data is presented in FIGS. 13 and 14. A coatingmade of yttria-stabilized zirconia powder was deposited in threedifferent thicknesses (3, 5 and 10 mils) on the fresh metal surface ofType 304 stainless steel (1/8 inch in diameter and 2 inches long) by airplasma spraying. The corrosion potentials of the zirconia-coatedelectrodes, a pure zirconium electrode and uncoated Type 304 stainlesssteel were measured against a Cu/Cu₂ O/ZrO₂ reference electrode in 288°C. water containing various amounts of oxygen. After the corrosionpotential measurement, test specimens were immersed in 288° C. watercontaining various water chemistry conditions for 3 months at opencircuit.

In the initial tests, YSZ-coated stainless steel electrodes were mountedin the autoclave along with a zirconium electrode, an uncoated Type 304stainless steel electrode and the reference electrode. All specimenswere immersed in pure 288° C. water at a flow rate of 200 cc/min for 2days. The corrosion potential was measured sequentially with incrementaladdition of oxygen, as shown in FIG. 13. At given oxygen levels up to200-300 ppb, the YSZ-coated electrodes showed low potentials (<-0.5V_(she)) essentially equivalent to those of the pure zirconiumelectrode, compared to the Type 304 stainless steel corrosion potentialvalues measured at the same level of oxygen. Further increase of theoxygen concentration increased the corrosion potential of the YSZ-coatedelectrodes.

After the system was left in 288° C. water containing various waterchemistry conditions for 3 months, the corrosion potential was againmeasured by increasing the oxygen concentration (see FIG. 14). This dataindicates that the corrosion potential behavior of the YSZ-coatedelectrodes was retained for extended periods.

From the foregoing data, it is apparent that the application of a YSZcoating on the surface of Type 304 stainless steel appears isadvantageous in maintaining a low corrosion potential (<-0.5 V_(she)) athigh oxygen levels (up to about 300 ppb), even in the absence ofhydrogen, by reducing mass transfer of oxygen to the metal surface andthereby mitigating SCC of the structural material. Since the oxygenconcentration during operation of a BWR is about 200 ppb, SCC in BWRstructural components could be mitigated by the application of a YSZcoating or any other electrically insulating protective coating on thesurfaces of the structural material.

The foregoing method has been disclosed for the purpose of illustration.Variations and modifications of the disclosed method will be readilyapparent to practitioners skilled in the art of water chemistry. Allsuch variations and modifications are intended to be encompassed by theclaims set forth hereinafter.

We claim:
 1. A method for mitigating growth of a crack in a surface of ametal component in a water-cooled nuclear reactor or associatedequipment, said uncoated surface of said metal component beingsusceptible to stress corrosion cracking in high-temperature water,comprising the step of applying a coating on said surface of said metalcomponent, said coating comprising electrically insulating materialhaving restricted mass transport crevices which penetrate to saidsurface of said metal component and which restrict the flow of oxidantsto said surface, whereby the corrosion potential of said surface of saidmetal component is decreased to a level below a critical potential atwhich stress corrosion cracking occurs.
 2. The method as defined inclaim 1, wherein said electrically insulating material compriseszirconia.
 3. The method as defined in claim 2, wherein said coatingfurther comprises zinc oxide.
 4. The method as defined in claim 1,wherein said electrically insulating material comprises yttriastabilized zirconia.
 5. The method as defined in claim 1, wherein saidelectrically insulating material comprises alumina.
 6. The method asdefined in claim 5, wherein said coating further comprises zinc oxide.7. The method as defined in claim 1, wherein said hydrogen is not addedto the feedwater of said reactor during reactor operation.
 8. The methodas defined in claim 1, wherein said metal component is made of stainlesssteel or other reactor structural material.
 9. The method as defined inclaim 1, wherein said electrically insulating material comprisesparticles sprayed onto said surface of said metal component.
 10. Acomponent of a water-cooled nuclear reactor or associated equipmentcomprising:a metal substrate having a surface which is susceptible tostress corrosion cracking in high-temperature water when left untreated;and a coating on said surface of said metal substrate, said coatingcomprising electrically insulating material having restricted masstransport crevices which penetrate to said surface of said metalcomponent and which restrict the flow of oxidants to said surface,whereby the corrosion potential of said surface of said metal componentis decreased to a level below a critical potential at which stresscorrosion cracking occurs.
 11. The component as defined in claim 10,wherein said electrically insulating material comprises zirconia. 12.The component as defined in claim 11, wherein said zirconia isstabilized with yttria.
 13. The component as defined in claim 10,wherein said electrically insulating material comprises alumina.
 14. Thecomponent as defined in claim 10, wherein said metal component is madeof stainless steel or other reactor structural material.
 15. Thecomponent as defined in claim 10, wherein said coating further compriseszinc oxide.
 16. The component as defined in claim 10, wherein saidelectrically insulating material comprises particles sprayed onto saidsurface of said metal component.
 17. A water-cooled nuclear reactorcomprising metal components which are susceptible to stress corrosioncracking during reactor operation and which have been treated tomitigate said stress corrosion cracking, each of said metal componentscomprising:a metal substrate having a surface which is susceptible tostress corrosion cracking in high-temperature water when left untreated;and a coating on said surface of said metal substrate, said coatingcomprising electrically insulating material having restricted masstransport crevices which penetrate to said surface of said metalcomponent and which restrict the flow of oxidants to said surface,whereby the corrosion potential of said surface of said metal componentis decreased to a level below a critical potential at which stresscorrosion cracking occurs.
 18. The nuclear reactor as defined in claim17, wherein said electrically insulating material comprises zirconia.19. The nuclear reactor as defined in claim 18, wherein said zirconia isstabilized with yttria.
 20. The nuclear reactor as defined in claim 17,wherein said electrically insulating material comprises alumina.